There are only a few recognised tests for diagnosing food allergies and intolerances:

Skin prick test

A few drops of the suspected food allergen are put on the skin which is then pricked. Food allergens that produce wheals at least 3mm larger than a negative control are considered positive. This test is 95% predictive of a negative reaction but a positive result doesn't necessarily indicate an allergy to a food. Foods should not be excluded on the basis of this test alone. The test is more useful for environmental allergies like animal hair.

The radio allergo sorbent test (RAST)

The RAST can be helpful in identifying food allergies by measuring specific IgE antibodies* in the blood. *IgE or Immunoglobulin E is a class of antibodies which are part of our immune system. The antibodies are produced by white blood cells in the body to fight infections from bacteria and viruses. IgE is the least common antibody in the blood but is responsible for triggering most allergic reactions. Allergic reactions to foods occur because the body mistakenly reacts to food protein (for example cow’s milk protein or soya protein) treating it as a hostile invader or allergen. The reaction causes the release of inflammatory substances in the body, like histamine, which result in allergic reaction symptoms like diarrhoea, wheezing and itchy red skin.

The RAST test is more expensive than a skin prick test and because the blood is tested the results take longer to come back. Interpretation of results can sometimes be difficult, but it can provide useful information in reaching a diagnosis when interpreted by an experienced clinician.

Exclusion diets

Exclusion diets are the treatment for specific food allergies and intolerances, but can also be part of the diagnostic procedure. Sometimes reactions to foods can be obvious and immediate and need to be excluded. In other instances it is uncertain which foods are causing a reaction and an exclusion diet needs to be followed for a period of time, with controlled food reintroduction to check for symptoms. Exclusion diets, particularly if there are a number of foods involved, should always be done under the supervision of a Registered Dietitian (registered with the Health Professions Council) who can give advice on ensuring the diet is nutritionally adequate. Exclusion diets can be a very useful way of diagnosing food allergies and intolerances.

Other allergy tests

With only a limited number of suitable tests for food allergies and intolerances it can be difficult to know who to turn to. Some GPs are inexperienced or unsympathetic and so it can be tempting to try out an alternative allergy testing centre. However these establishments and the tests that they offer cannot be recommended as useful tests in the diagnosis of food allergy and intolerance.

Intradermal tests have a high false positive rate and are not recommended by medical authorities. Other tests which include applied kinesiology, hair testing, electrodermal tests, tests to determine specific IgG antibodies* in the blood, cytotoxic tests, iridology, vega testing and sublingual provocative food testing have not been shown to be helpful in diagnosing food allergy and intolerance. A Consumer Association report found that known allergies were missed and other allergies were exaggerated, potentially resulting in inadequate diets with unnecessary dietary restrictions. *IgG or Immunoglobulin G is another type of antibody which attacks bacteria and viruses in the body. It is the most common antibody in the body but appears to have a more minor role than IgE in allergic reactions.